Here's why multiple disks are strongly recommended, including separate disks for the projects and exports:

The SATA interface that all lower-end and smaller systems use is only half-duplex, which means that data transfers can travel in only one direction at at time. And an assigned block of transfers in one direction must be completed before a block of transfers can start traveling in the opposite direction. However, if you use the same disk for both the projects and the exports, then the operation will now require simultaneous reads and writes which no SATA device is capable of performing. Fortunately, the project files tend to be on the small side, which minimizes the performance loss on export.

With that stated, the idea to have separate disks is to avoid having the media cache on the same disk as the projects, which will slow down the editing and export performance significantly.